Combined Multiple Memories

This invention relates in general to the field of memory systems, and more particularly, to a combined memory system which merges pitch matched arrays of different types of memory, for example, Random Access Memory (RAM), Read Only Memory (ROM), Electrically Programmable ROM (EPROM), or some combination thereof.

Many electronic circuits, typically microprocessor based circuits, are required to store digital information either temporarily or permanently, and must further access the information quickly. Furthermore, it is common for many electronic circuits to require both temporary and permanent data storage in the same system. The number of bits required for storage and thus the number of memory cells may vary considerably from system to system. Because the number of storage bits required may be large, it is advantageous to increase the density of memory bits for a given area.

Application Specific Integrated Circuits (ASICs) is an example of an application that often requires different combinations of memory, and each type of memory in different amounts. The memories are typically each placed in separate locations on the chip, each being a stand alone memory system. Typically each memory consists of memory cells arrayed in an x-y array having row decoders located along the y axis to provide word decoding and having column decoders located along the x axis for providing bit decoding. The memory cells of each memory array each have an x and y pitch (width and height of each cell, respectively) which are typically matched to the y pitch of the row decoders and to the x pitch of the column decoders, respectively.

Since the array of the memory cells normally takes up the majority of area, the memory cell is designed first to be as compact as possible both in the x and y pitches. The row and column decoders are then designed to match the already determined x and y pitches of the memory cells. This is an efficient and effective method of design, especially in large memories. In ASIC applications, where the number of memory cells generally are not large, the row and column decoders take a proportionately larger amount of area. Furthermore, Random Access Memories (RAM) cells are typically larger than Electrically Programmable Read Only Memory (EPROM) cells which in turn are larger than Read Only Memory (ROM) cells.

ASIC applications which require differnt types of memories in varying amounts, often are not required to access the different memories on the same clock cycle (or asynchronously). The different types of memories may then be accessd individually, usually putting their addressed contents onto a bus. In effet the row decoders, and in some cases the column decoders of each memory array are redundant and moreover require a significant amount of area. If the smaller memory cells were designed to be pitched matched to the largest memory cell, then the row and/or column decoders could be shared. This is especially efficient when there are smaller amounts of the smaller memory cells.

Pitched matched arrays may be configured using conventional Computer Aided Design (CAD) techniques. However hierarchical structured design methods, and more particularly, silicon compilers, having made it more efficient in laying out such an integrated circuit design. The silicon compiler may take a high level description of the system and convert it into a parameterized layout of the circuit. Although the initial preparation of the design software may consume a lenghty time period, subsequent circuit design for multiple circuits requires substantially less time. Once the x and y pitches of the largest memory cell are determined, these parameters may be used to quickly layout other pitch matched memories.

Thus, what is needed is a combined memory system which merges pitched matched arrays of different types of memory, for example, Random Access Memory (RAM), Read Only Memory (ROM), Electrically Programmable ROM (EPROM), or some combination thereof.

Accordingly, it is an object of the present invention to provide an improved memory system.

It is further an object of the present invention to provide a memory system having different memory arrays sharing a common decoder.

It is yet a further object of the present invention to provide a combined memory system having pitch matched arrays.

In carrying out the above and other objects of the invention in one form, there is provided a monolithically integrated memory system comprising a row decoder coupled for receiving an address for providing a plurality of word lines wherein only one of the word lines is active at any given time. The row decoder includes a plurality of decoder cells that are pitch matched to the largest memory cell of multiple but different memory arrays. A first memory array for storing digital information is coupled to the row decoder for receiving at least one of the plurality of word lines, the first memory array having a plurality of cells, each of the cells having an x and a y pitch. A second memory array for storing digital information in a manner different from the first memory array is coupled to the row decoder, the second memory array having a plurality of cells being spaced and aligned with respect to the first memory array and, each of the cells having an x and a y pitch, wherein the x pitch is different than the x pitch of the cells of the first memory array and the y pitch of the cells of said first and second memory arrays are substantially equal.

The above and other objects, features, and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of a Static RAM cell and a ROM cell.

FIG. 2 is a block diagram of the present invention.

FIG. 3 is an alternative block diagram of the present invention.

In FIG. 1 a Random Access Memory (RAM) cell 1 is compared to a Read Only Memory (ROM) cell 2. The RAM cell 1 normally contains four to six devices while the ROM cell 2 normally contains one device. The RAM cell 1, therefore, typically consumes about four times the area as that of the ROM cell 2 for a given technology. Furthermore, the RAM cell 1 requires a wordline and a wordline for selection and outputs a bit and a bit. The ROM cell 2 requires only a wordline for selection and outputs only a single bit. The width of the RAM cell 1 is shown by the distance x which is the x pitch, and the height is shown by the distance y which is the y pitch. Similarly the x and y pitches of the ROM cell 2 are shown by x1 and y1, respectively. While the RAM cell 1 is significantly larger than the ROM cell 2, the pitches y and y1 may be made to match by increasing the x pitch and decreasing the x1 pitch. It is further possible to include two or four bits of ROM in the ROM cell 2 with additional decoding being done at the respective column decoders in order to more efficiently match the areas of the RAM and ROM cells 1 and 2. The RAM and ROM cells 1 and 2 are given only as an example as the pitch matching is applicable to all types of memory cells.

FIG. 2 shows a partial memory system 3 wherein a RAM array 4, a ROM array 5, and an Electrically Programmable ROM array 6 have been combined. The RAM array 4 comprises a plurality of RAM cells 1, the ROM array 5 comprises a plurality of ROM cells 2, and the EPROM array comprises a plurality of EPROM cells 21. Address buffers 7 receive an address for selecting a particular bit of memory from one of the memory arrays 4, 5, or 6. The address buffers 7 are connected to a row decoder 8 by a bus 17 having n bits, and a column decoder 11, a column decoder 12, and a column decoder 13 by a bus 18 having n1 bits. The Address buffers 7 provide additional drive for address signals on the busses 17 and 18. The row decoder 8 is made up of a plurality of decoder cells 19, the row decoder 8 being connected to the RAM array 4, the ROM array 5, and the EPROM array 6 by a plurality of wordlines 9.

Each decoder cell 19 is pitch matched in the y direction to the RAM cell 1 which is further pitch matched to the ROM cell 2 and the EPROM cell 21 in the y direction. The x pitches of the RAM cell 1, the ROM cell 2, and the EPROM cell 21 are not typically pitch matched. A plurality of bit lines 25 from the RAM array 4 are connected to the column decoder 11. The column decoder 11 comprises a plurality of decoder cells 22 wherein the decoder cells 22 are typically pitch matched in the x direction to the RAM cells 1. Similarly the ROM array is connected to the column decoder 12 by a plurality of bit lines 23, and the EPROM array 6 is connected to the column decoder 13 by a plurality of bit lines 24.

When the address buffers 7 receive a new address, the address signals on the busses 17 and 18 are decoded by the row decoders 8 and the column decoders 11, 12, and 13, respectively. As a result, one of the plurality of wordlines 9 will become active and a row each of the RAM cells 1, the ROM cells 2, and the EPROM cells 21 will be selected. Simultaneously, the column decoders will select a plurality of bits from one of the RAM cells 1, the ROM cells 2, or the EPROM Cells 21. It is not necessary to have the same size words (same number of bits per wordline) for the RAM array 4, the ROM array 5 or the EPROM array 6 since the column decoders 11, 12, and 13 may have varying numbers of decoder cells. The RAM cells 1, ROM cells 2, and EPROM cells 21 are used here as an example several alternative memory types could be used in their place.

An alternative structure of a combined memory array 14 is shown in FIG. 3. The numbers used in FIG. 2 are repeated in FIG. 3 to depict equivalent structures. The connections are the same in FIG. 3 as in FIG. 2 with the exception of a 3 to 1 multiplexer 15 and a column decoder 16 which replace the column decoders 11, 12, and 13. The 3 to 1 multiplexer 15 is connected in the same manner as the column decoders 11, 12, and 13 of FIG. 2 but has a plurality of bits 26 at its output connected to the column decoder 16. In this arrangement, area is saved by the simpler 3 to 1 multiplexer 15 and the single column decoder 16. However, this requires equal size words from each of the RAM array 4, the ROM array 5, and the EPROM array 6.

In FIGS. 2 and 3, it is not necessary to have equal number of bits per array. Nor is it necessary to have an equal number of words per array. If for example, the RAM array 4 had 200 words and the ROM array 5 had only 100 words, then the bottom half of the plurality of wordlines 9 would only be connected to the RAM array 4 and not to the ROM array 5. Significant savings in area is realized by combining the memory arrays since only one set of row decoders is required. Also area is saved by removing the need for redundant power bussing as well as redundant word and bit lines.

Building the combined multiple memories may be automated by the use of a silicon compiler. Compiler cells may be defined for the different memory arrays wherein the input parameters include the number of memory cells, the number of rows and columns, and the x and y pitches of the memory cells. The memory array having the largest memory cell would be layed out first to have a minimum y pitch. The remaining memory cells for each additional array would then be layed out to have the same y pitch while the x pitch is variable. Next the row decoders would be layed out to have a matching y pitch and a variable x pitch. The following step includes the addition of the column decoders that would be x pitch matched to the memory cells of their respective memory arrays. Finally the arrays and decoders would be placed and interconnected according to the common design rules.

It can be appreciated by one skilled in the art that the variety of memories that may be combined into one pitch matched system is limited only by the ability of the technology to concurrently implement the different memories on one substrate. Additional memories may include, for example, Electrically Erasable Programmable Memory (EEPROM), Programmable Read Only Memory (PROM) and Electrically Alterable Read Only Memory (EAROM).

By now it should be appreciated that there has been provided a combined memory system which merges pitch matched arrays of different types of memory. For example, Random Access Memory (RAM), Read Only Memory (ROM), Electrically Programmable ROM (EPROM), or some combination thereof.